The present invention relates to a method of forming an image with magnetic toner directly on a surface of a recording medium by means of a large number of needle electrodes operable in response to an electric image signal.
Conventionally, the formation of an image in an electrophotographic apparatus is generally carried out by using a photosensitive drum as an image-bearing medium, uniformly charging the surface of the photosensitive drum by means of a corona discharge unit, exposing the photosensitive drum surface to form an electrostatic latent image, bringing a magnetic developer into contact with the image-bearing photosensitive drum surface by a magnetic brush method, etc. to form a toner image, transferring it onto a recording sheet, and fixing the toner image thereon.
Recently, however, a method of forming a visual image with toner directly on a dielectric member made of alumite or other materials by a plurality of needle electrodes without using a photosensitive drum was proposed (for instance, see U.S. Pat. No. 3,816,840). Specifically speaking, a drum made of aluminum, etc. and coated with an alumite layer of about 10 .mu.m in thickness contains a permanent magnet near an inner surface of the drum, and a container containing conductive magnetic toner is arranged near an upper part of the drum in opposition to the permanent magnet in the drum. Part of this toner container is provided with a magnetic plate and a plurality of needle electrodes opposing to the drum and the permanent magnet contained therein. A magnetic field generated by the permanent magnets serves to form toner chains between the alumite layer and the electrodes, bringing part of the toner chains into contact with the alumite layer. Because of this arrangement, when an image-forming electric signal, for instance, a pulse voltage of about 50V is selectively applied to a plurality of needle electrodes, the toner being in contact with the alumite layer receives a Coulomb force, and the toner is selectively attracted onto the alumite layer on the outer surface of the drum under this Coulomb force while the drum is rotating, thereby effecting development. The toner image is then electrostatically transferred onto a plain paper and then fixed thereon to provide a copy image.
It has been found, however, that various problems take place when conventional magnetic toner is used without any treatment to form a toner image directly on a dielectric medium as mentioned above. Specifically, with conductive magnetic toner (for instance, U.S. Pat. Nos. 3,639,245, 4,189,390 and 4,482,623), a toner image is blurred when transferred because such magnetic toner has a low resistivity (bulk resistivity of 10.sup.2 -10.sup.3 .OMEGA..cm or so). On the other hand, when semiconductive or insulating magnetic toner is used, a high image density cannot be obtained when developed, because the toner has a high resistivity of 10.sup.9 -10.sup.16 .OMEGA..cm or so. Conductive fine particles such as carbon black can be added to magnetic toner particles to decrease the internal resistance thereof, but this solution requires a large amount of carbon black, leading to extremely poor fixability.